It’s not often the secretary of defense makes it a point to visit U.S. nuclear weapons sites, but SECDEF James Mattis will do so this week in what some are interpreting as a signal to one of America’s potential challengers.

Just days after North Korea tested what analysts now think was a 250-kiloton nuclear device, Mattis will tour two key U.S. nuclear weapons bases as American forces in the Pacific remain on high alert.

Mattis will visit Minot Air Force Base in North Dakota, which controls ICBMs scattered across the state and is home to B-52 bombers, which can launch nuclear cruise missiles. He will also visit U.S. Strategic Command, in Omaha, Nebraska, which would control a nuclear war.

“His visit to two nuclear-focused locations comes at an opportune time given the ongoing Nuclear Posture and Ballistic Missile Defense Reviews,” Pentagon officials said in a statement late Tuesday.

President Donald Trump ordered a review of America’s nuclear forces shortly after taking office in January. That said, Mattis’ visit is meant, in part, to demonstrate to North Korea the United States’ much larger, much more lethal, and far more advanced nuclear capability.

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